DEAR DOCTOR K: As I've entered my 40s, the skin on my hands and face has started to change. What happens to our skin as we age?

DEAR READER: The shortest answer is that our skin gets old, like the rest of us. In fact, the three layers of skin get old in different ways.

The skin's outermost layer, the epidermis, forms a protective physical barrier. The dermis, or middle layer, contains collagen and elastin, which provide strength, firmness and elasticity. It also contains blood vessels, immune cells, nerves and glands that produce sweat and oil. The deepest layer consists of connective tissue and fat. (I've put an illustration of these layers on my website, AskDoctorK.com.)

As the years go by, skin undergoes a number of changes. Epidermal cells don't slough off as easily. Skin doesn't retain as much moisture. The collagen and elastin in the middle layer break down. As a result, the skin is less firm and less elastic.

It's similar to what happens to a rubber band as it gets old and dry: It loses elasticity. As a result, when gravity tugs on the skin, instead of bouncing back, it starts to sag: Fine lines form around the eyes, deepened lines appear at the corners of the mouth and across the forehead, and skin in various places (such as under the jaw and in the neck) starts to hang down.

Other changes may not be as noticeable to you. For example, the skin's ability to fight infection, feel sensations and regulate body temperature also diminishes with age.

But the single biggest cause of damage to skin as you age is not aging itself; it's sun exposure. Over the years, sun exposure causes fine and coarse wrinkles; baggy skin with a yellow, leathery appearance; and dry, scaly skin. It also increases the risk for skin cancer.

I grew up near the beaches of Southern California. Many sun worshippers were on the beach pretty much all day, every day. By the time they were in their late 30s, many of these people had skin that was as dry, scaly and wrinkled as someone in his or her 70s.

To distinguish the effects of sun exposure from those of aging, look at and feel the skin on your face and the backs of your hands. Now do the same on a part of your body that isn't regularly exposed to the sun, such as your lower abdomen or buttocks. The difference can be dramatic.

It's never too late to protect your skin from sun damage:

• Use a sunscreen with an SPF of 30 or higher.

• Reapply sunscreen every couple of hours.

• Wear wide-brimmed hats and other protective clothing.

• Avoid the sun between 10 a.m. and 2 p.m., when its rays are most intense.

It's true that our skin ages, and there's not a lot we can do about it. But we can do something about the damage done by the sun that makes our skin — and us — look old.